With increasing market pressure for smaller, faster and more sophisticated end products using integrated circuits, the electronics industry has responded by developing integrated circuits which occupy less volume, yet operate at high current densities. Power supply assemblies for such microprocessors generate considerable heat during operation. If the heat is not adequately removed, the increased temperatures generated by the power supply assemblies will result in damage to the semiconductor components.
A heat sink is commonly used to transfer the heat away from the power supply or other heat generating assembly. The heat sink generally includes a plate or body formed from a conductive metal, which is maintained in thermal contact with the assembly for dissipating heat in an efficient manner. Fins optionally protrude from the plate for providing an increased surface area for heat dissipation to the surrounding environment.
The current industry technique for providing thermal contact between a microprocessor power supply assembly and a heat sink is to interpose a thermal interface material between the two, which facilitates heat transfer from the active device to the heat sink.
One method is to apply a ceramic filled thermal grease, which is typically silicone based, between the heat sink and the power supply. Thermal greases provide excellent thermal properties, but require an extensive assembly process with high manufacturing cost. The product is usually applied by hand, from a syringe, or with an aluminum carrier. This process is labor intensive and slow and does not lend itself to automation.
Another method for providing a conductive interface includes the use of thermally conductive wax compounds. These compounds, however, are generally brittle at ambient temperatures and easily chipped off, resulting in a high thermal resistance. The low viscosity of the wax at operating temperature causes the wax to flow out from between the active component and the heat sink, resulting in a high thermal resistance. Further, because of the brittle nature of the wax compounds, they are difficult to manufacture and apply to a heat sink.
Thermally conductive silicone rubbers have also been used as conductive interfaces. Although soft and pliable, the silicone rubber requires relatively high pressure and a long warm-up time to attain a low thermal resistance. The rubbers have poor flow characteristics which result in a low thermal conduction when there is a mismatch of flatness between the heat sink and the heat producing device. Differences in the thermal coefficient of expansion between the silicone rubber and the heat sink can result in high thermal resistance during temperature cycling. These effects lead to a poor thermal conductivity from the heat producing device to the heat sink.
Thermal interface materials often combine a thermally conductive filler material with a matrix of wax, grease, or polymeric material. U.S. Pat. No. 4,869,954 to Squitieri, for example, discloses a cured, form stable material for use in removing or transferring thermal energy. A urethane binder is filled with a thermally conductive material, such as boron nitride or magnesium oxide. The filled binder is used by itself or in conjunction with a reinforcing substrate for dissipating heat from an electronic component or device.
U.S. Pat. No. 5,194,480 to Block et al. relates to thermally conductive electrically insulating filled elastomer compositions including a thermoplastic rubber, which may be crosslinked and fillers such as boron nitride and alumina. The filled elastomer can be molded or pressed into desired shapes.
U.S. Pat. Nos. 5,454,473 and 5,591,034 to Ameen et al. disclose a thermally conductive interface material for thermal conduction between electronic components comprising an open structure fluoropolymer material, such as expanded polytetrafluorethylene, with uncoated thermally conductive particles attached to solid portions of the polymer.
U.S. Pat. No. 6,054,198 to Bunyan et al. discloses heat transfer materials applied to a film or release sheet. The thermally conductive materials comprise waxes, thermoplastics, or acrylic pressure sensitive adhesives, in combination with a filler, such as boron nitride.
Such materials provide a relatively high thermal conductivity by means of thermally conductive particles dispersed within a polymer matrix. However, none of these materials is able to attain the theoretical limit of conductivity that the fillers possess, primarily because of the high interfacial thermal resistance between the particles and the polymer matrix in the heterogeneous mixture.
Heat transfer members have also been formed from soft metals, such as indium or gallium alloys. See, for example, Yamamoto, et al., U.S. Pat. No. 4,729,060; Altoz, U.S. Pat. No. 4,915,167. Other thermal interfaces employ polymeric thermally conductive cure-in-place compounds. These compounds are rigid after cure. For example, Huang, et al., U.S. Pat. No. 5,062,896 discloses electroconductive solder compositions formed from powdered eutectic metal alloys, such as bismuth/tin alloys, in a polymer, such as a thermoplastic poly(imide) siloxane, which are subsequently cured. Themoset epoxy compounds filled with silver are also disclosed. Such compositions have a poor reliability because of a difference in thermal coefficient of expansion between the material and the heat sink, causing cracks and failure during temperature cycling. The polymeric materials are labor intensive to apply and require long cure times.
U.S. Published App. 20010038093 discloses compliant crosslinkable materials, such as silicone resins, in which a solder material, such as an indium tin complex, is dispersed.
The present invention provides for a new and improved thermal interface material, which overcomes the above-referenced problems and others.